by Louise-Océane Delion, marine biologist and scientific content creator for Koraï
Coral reefs are disappearing globally at an alarming rate due to human activities. Already 50% of the coral reefs are gone, and up to 90% will disappear within 50 years if we don’t do anything about it.
Africa is far from exempt when it comes to the loss of coral reefs. A recent study* showed that all coral reefs in Eastern Africa are vulnerable to collapse: they will decline to such a point that the whole ecosystem will be functionally extinct, without the capacity to recover or adapt to future conditions. This will have tremendous effects on all biodiversity found within the reefs but also on human populations who rely on these ecosystems for protection, livelihoods and food.
Our mission at Koraï is to restore African marine ecosystems, starting with coral reefs in Madagascar. As we start to restore coral reefs in this region, we need to understand what is happening there. Dive with us to understand how science shows us that coral reefs are about to collapse in Africa and how you can embark on Koraï’s mission to restore coral reefs in Africa.
Where are we?
Although less famous than the Great Barrier Reef or the Caribbean, the Western Indian Ocean is one of the hotspots of marine biodiversity.
This region is - like its name suggests - the western part of the Indian Ocean, embracing Eastern Africa. Coral reefs are distributed in shallow waters of different countries such as Tanzania, Kenya, Mozambique, Madagascar*, Seychelles, the Mascarene Islands and Delagoa, representing a surface of 11,919km2 and accounting for 5% of global coral reefs (the Great Barrier Reef accounts for 10% of the world's corals).
*Our first Koraï coral nursery is located in Northern Madagascar!
Behind the (scientific) scenes
How do scientists actually measure the health of coral reefs and determine that they are disappearing? We often read big apocalyptic titles stating that global reefs are doomed to disappear, but few of us understand the science behind it. Let’s try to approach it in a straightforward way.
Most of the time, the health of coral reefs is based on one key indicator: the live coral surface area.
Easy. But too simple.
When talking about the health of coral reefs, it is important not to focus only on the corals themselves because coral reefs are more than just corals: they are ecosystems. Corals create the foundation for a community in which many other species interact: algae, fish, invertebrates, turtles… This combination of species makes the ecosystem rich and diverse. Their interaction determines the state of the ecosystem, with certain species controlling others through competition, predation or even symbiosis.
In this case, it is important to take an ecosystem point of view to measure the health of the coral reefs. Scientists are using a framework developed by the International Union for the Conservation of Nature (IUCN) which uses different criteria to define the health of the ecosystem and hence, the level of threat it is facing.
Here are a few criteria that can be used to measure how threatened coral reefs are :
A reduction in geographic distribution
The decline in the extent of an ecosystem is a direct measure of its damage. 10% of known coral reef cover is the threshold below which the reef is considered in danger. Below 5% left, the coral reef is near collapse with poor recovery possible.
The smaller an ecosystem is, the more vulnerable it is: any given threat (bleaching event, pollution event…) will affect a larger proportion of the overall ecosystem extent.
These two criteria, however, only talk about the size of the ecosystem and are not really taking an ecosystem's point of view. But here are more :
Degradation in the quality of abiotic (physical) factors
One of the most important factors for coral reefs is the sea surface temperature. An increase in water temperature can cause thermal stress on the corals and lead to bleaching events, where the algae which they live in symbiosis with get expelled, leading to the death of the corals.
The number of bleaching events and the number of weeks with above-average water temperature can be used as indicators to determine the level of environmental stress affecting the coral reefs.
Changes and disruption in other species (biotic factors)
As mentioned before, an ecosystem is made of many species that all interact with each other and as a result, have an impact on the whole.
A disruption in species interactions can lead to changes in an ecosystem up to a point where some organisms become too abundant and start to threaten the global health of the system.
In the case of coral reefs, measuring the fleshy algae-coral cover ratio, or the abundance of herbivorous fish can be key indicators: an excess of algae shows that the ecosystem is shifting from a diverse coral reef to a less diverse algae bed.
Once measurements are being done for each of these criteria, it is possible to follow a grid and categorise the level of vulnerability of the ecosystem: from least concern to critically endangered.
The big threats to African corals
By using the criteria aforementioned and analysing them, scientists have drawn a terrible conclusion: coral reefs are vulnerable to collapse in all regions of the Western Indian Ocean.
Globally, many threats are affecting coral reefs, from local ones such as fishing, pollution, diseases and cyclones, to global ones such as warming and acidification.
In this region of the world, scientists were able to determine the two dominant pressures that are acting on coral reefs: overfishing and climate change, each of them affecting different regions differently.
For example, in the islands such as Madagascar, Comoros, Seychelles and the Mascarene Islands (Mauritius and Reunion), future warming is the highest threat to coral reefs.
With climate change, the seawater temperature is rising causing thermal stress and bleaching events as mentioned before. The damage caused by climate change has already been observed: previous records have shown a drastic decline in coral reefs since 1998 in the region, and some massive bleaching events happened in 2016.
In continental regions such as Tanzania, Kenya and Mozambique, overfishing is the predominant threat. The decline in fishes, herbivores and piscivores, will change the dynamics of the ecosystem, causing an imbalance between the corals and the algae beds, subsequently impairing the health of the whole ecosystem.
The collapse of the reefs affects us all
« Collapsing » sounds rather apocalyptic… but it’s important to understand the depth of this term and to distinguish it from « declining ».
If coral reefs are declining, we can assume it is about their size. However, if coral reefs are collapsing, we can understand that it is not about the corals themselves but about the ecosystem as a whole.
If corals are disappearing, all the species living within the system they create will be impacted. As coral reefs act as nurseries, refuges, and foraging grounds and sustain about 25% of marine life, we can imagine how many species will be affected too. The collapse of coral reefs implies the collapse of most marine life.
And it’s not just about species being lost, it is also about the potential adaptation, recovery and resilience of the ecosystem that is being impaired, making it potentially doomed forever.
We too, will be affected. Let’s remember that we, humans, depend on coral reefs for many reasons: they protect our coastlines from erosion and storms, they provide us with food, and they sustain eco-tourism activities. They are essential to our livelihoods and way of life on this planet. If coral reefs are collapsing, human societies too, will be greatly affected.
What can we do?
The conclusion of this scientific study is quite grim. If the current trends of climate change and fishing efforts keep the same way, all coral reefs from the Western Indian Ocean will collapse in 50 years.
But then, what? What do we do with this information? Do we close the internet, keep on with our lives and wait for the disaster to happen? Or do we realise we have the responsibility and possibility of doing something to limit the damage?
Damage will happen, yes. But to what extent? Our aim is not to diminish the sense of emergency but rather to use it as a wake-up call to foster all the efforts to save what we can.
There are many tools available to help us try to « fix » the situation. Reducing our CO2 emissions to limit climate change and enforcing better management of fishing practices is the first step. Designing and implementing marine protected areas to create refuges of biodiversity and allow marine life to reproduce, evolve and adapt with limited human impacts, is also a necessary step.
And then we can help to boost coral reefs through restoration. We can contribute to rebuilding coral reefs that are already damaged whilst helping them to adapt to future conditions in the ocean by conducting efforts of restoration in a climate-resistant way (for example by selecting better, more high-temperature resistant individuals…).
Although the situation can seem terrifying and overwhelming, we also need to realise that in 50 years, a lot can be done, especially if appropriate and sufficient resources are being deployed. Successful examples of coral reef restoration around the world are showing us that all is not lost and that if we take action now, we can save the reefs. This is why Koraï exists: to foster the restoration of marine ecosystems in Africa, starting with coral reefs in Madagascar; and we need you to join us on our quest.
OBURA, David; et al. Vulnerability to collapse of coral reef ecosystems in the Western Indian Ocean. Nature Sustainability, 2022, vol. 5, no 2, p. 104-113.